In the prior art, variable pitch propeller systems typically utilize either hydraulic or mechanical activating systems. In either instance, the systems are very complicated and expensive when used for marine propulsion. Additionally, backfit of either hydraulic or mechanical variable pitch propeller technology to a ship which has a fixed pitch propeller is difficult and expensive if at all possible.
A typical hydraulic system generally includes a hydraulic pump located within the ship, actuating means disposed within the interior of the propeller hub, and hydraulic fluid pumped by the pump via rotary fluid couplings through the multiple passages in the interior of the propeller shaft to the actuating means. The major disadvantage of the hydraulic system is the difficulty of providing a porting system through the shaft. The propeller shaft must be multiply drilled along its length to accommodate the porting, rotary fluid couplings must be provided to port fluid from the dedicated hydraulic plant to the propeller shaft, and, if the propeller shaft is multipart, a sealing system for the individual ports must be included at those points where the shaft pieces are coupled. The actuating means in the propeller hub includes a hydraulic ram and blade actuating mechanisms. These parts require precision fitting since the hydraulic forces on each side of the ram head must be balanced to provide the correct propeller pitch. Precision fitting of these parts can be difficult to achieve due to the large size of these parts in the propeller hub.
A mechanical variable pitch control mechanism requires a mechanical actuating rod extending the length of the shaft through a single hole in the center of the propeller shaft to the blade rotating mechanism. Although providing the single hole in the shaft for the mechanical system is preferable to the complicated porting system required of the hydraulic system, there are disadvantages here as well. To permit access between the actuating rod and the actuating mechanism located within the ship, the mechanical actuating rod must be longer than the propeller shaft to enable it to protrude forwardly from the reduction gear casing. Furthermore, mechanical systems are typically not utilized on direct drive ships such as a diesel, since there is no point of access to the end of the propeller shaft.